Many natural resource industry activities involve determining the unknown or generally uncertain condition of a well having a metallic casing or other tubing strings cemented or hung in a wellbore. Tubing strings are used to handle fluids produced from or injected into an underground formation. Certain formation (or other) fluids, such as geothermal fluids, may present especially difficult corrosion and other fluid handling problems. Therefore, multi-layered construction (e.g., interior and exterior barrier layers composed of a fluid resistant material such as cement or concrete covering a steel pipe) may be used to handle difficult, corrosive, erosive and scaling fluids.
The presence or absence of fluids behind the tubulars of the underground well should be determined periodically to safely operate a well in a difficult application. Even if tubulars are cemented to a formation, formation fluid behind a cemented liner or a cemented casing can permeate the cement and corrode the liner or casing and cause cement failure, resulting in possible unsafe operation. Since removal of a cemented casing is impractical, in-situ inspection of tubulars and fluids is required. Even when tubulars are not cemented, in-situ inspection is preferable.
In-situ well inspections are currently accomplished using various logging tools and instruments. Some instruments project pressure (e.g., ultrasonic) or electromagnetic waves spherically/radially outward from near the tubular axis and detect returning waves (i.e., waves affected by the casing/cement materials and interfacial conditions). The instruments detect the induced or reflected signals/waves (i.e., perturbed signal) resulting from an interaction between the generated signal and the surrounding materials. The detected perturbed signal indicates and represents one or more casing or cement conditions. Some instruments are supported by a wire line and have two or more receivers placed at different locations. Other instruments may have different orientations for transmitter(s) and receiver(s) so as to obtain radial and/or directional information on conditions at a given depth. Material types and interfaces are detected by "known" changes in the perturbed (reflected or induced) signals.
Normally, the initial (i.e., first radially outward) material/interfacial surface most strongly perturbs the signal, especially for sonic signals. Signals coming from subsequent interfaces (i.e., surfaces) or materials must be corrected for the perturbations caused by the initial material and interface (i.e., subsequent return signals must pass through intermediary signal perturbing materials/surfaces after first passing through them to reach the outermost surface of interest). Thus, unless the initial materials and surfaces do not affect the wave (i.e., are relatively transparent to the signal), the condition of the exterior of a casing or formation radially outward from the casing must be determined from signal analyses which correct for known signal perturbing intermediary material/interfaces.
Besides the effects produced by the presence of intermediary materials of interest, existing instruments detect other signal perturbing influences. These influences include those identified with fluids present within the tubing or casing, fluids occupying the annular spaces between tubing and casing strings, fluids between casing strings, and fluids between a casing string and the bore hole. Other influences include the underground formation, formation fluids, and the formation geometry. These fluid and other perturbing influences must also be considered in the analysis of the detected signal to accurately determine the presence or absence of fluid contacting the outward facing surface of the tubing or cement.
Existing detected signal analysis methods generally use an idealized model with assumptions. The model treats the detected signals as perturbed by nominal or ideal signal perturbing conditions. For example, an idealized data analysis may assume a perfectly cylindrical steel casing, a composition of a wellbore fluid, and a homogeneous and infinitely thick formation layer (i.e., data are not affected by the formation boundaries). These models then provide idealized corrections for "known" perturbing factors.
However, the discrete surface or condition affected components of a signal (i.e., indicators) may be hidden in the data (e.g., signal characteristics obscured by a low signal to noise ratio) in difficult applications. In addition, some perturbing influences or factors may not be "known," making proper corrections difficult or impossible. For example, a deposit of a magnetically active scale having unknown magnetic properties will perturb induced electromagnetic signals carrying information with respect to a metallic casing wall. Failure to accurately identify or "know" of the presence of an intervening signal perturbing material(s) compromises all other data. This has led to the inadequate detection of unsafe in-situ casing and cement conditions caused by the presence of fluids.
All of the current in-situ analysis methods known to the inventors to detect the presence or absence of external fluids either will not work when an unknown intervening material is present, or they require a distinct intervening material signal to correct the perturbed signal, or they require a signal which is not affected by intervening materials, or they require an independent knowledge of factors which perturb the signal.